Smith won his first professional kickboxing title in 1983 for the World Kickboxing Council by defeating Light Heavyweight Champion Tony Morelli by knockout in the seventh round. Later that year, he won the World Kickboxing Association Heavyweight Championship from Travis Everett with a KO via low kicks. Smith then went ten straight years without a loss.

He also had a notable win in 1991 over Stan Longinidis who, at the time, was tearing through the ranks and was regarded as a super up and coming fighter. Maurice won this 12 round match by split decision after coming back from a knockdown in the first round by Stan and surviving an early onslaught.

In 1993, Smith was invited to the K-1 Grand Prix '93 along with seven of the world's best Light Heavyweight and Heavyweight kickboxers. Smith won his first match by defeating Japanese fighter Toshiyuki Atokawa in the quarterfinals by unanimous decision. In the semifinals he met Dutch fighter and future K-1 legend, Ernesto Hoost, where after a hard fought battle Smith was knocked out by left highkick in the final round, ending his tournament.

After a stint in Japanese promotions Pancrase and RINGS, accumulating a 4-7 record, Smith joined Battlecade Extreme Fighting in 1996 to fight against the Heavyweight Champion Marcus "Conan" Silveira. Throughout the fight Maurice showed excellent use of the sprawl technique which negated the Brazilian jiu-jitsu fighter's attempts at takedowns. Smith then knocked out Silveira in the third round with a kick to the head. In doing so, he became the Extreme Fighting Heavyweight Champion and became the first legitimate striker to win a top-end MMA event.

He then defended his title in the fourth show against judo exponent Kazunari Murakami with a first-round knockout. After the win, the company folded and Maurice joined the largest MMA promotion in the United States, the UFC.

Smith developed a close relationship with Frank Shamrock, who he met while training at the Lion's Den. Shamrock and Smith worked synergistically; Shamrock, a submission specialist, improved Smith's ground game while Smith, in return, improved Shamrock's striking. Smith and Shamrock eventually formed their own MMA team, called the Alliance.

At UFC 14 on July 27, 1997, Smith faced UFC Heavyweight ChampionMark Coleman in a title fight. Smith was considered to be a massive underdog for the bout, but shocked the mixed martial arts world with a unanimous decision victory to win the UFC Heavyweight Championship. With this win, Smith became the first striker to survive the attack of a world class wrestler. Through the use of an active guard, Smith was able to negate Coleman's vaunted "ground and pound" attack. After nearly 15 minutes of attempting to damage Smith through his guard Coleman was exhausted, allowing Smith to capitalize on the feet. He then successfully defended his belt against David "Tank" Abbott winning via strikes due to leg kicks, before losing his belt to MMA legend Randy Couture by majority decision, in a close fight.

Smith returned to MMA on March 30, 2012 at the second Resurrection Fighting Alliance (RFA) card against Jorge Cordoba, his first career fight as a Light Heavyweight. After spending the first two rounds picking apart Cordoba with various strikes, Maurice finished the fight with a head-kick knockout. He was scheduled to fight Ryan Lopez on June 30, 2012 at Resurrection Fighting Alliance's third event, RFA 3.[2] Weeks before the fight, though, Lopez was forced to pull out of the fight due to an unspecified illness. This would have been Smith's second career fight at Light Heavyweight.

Smith returned to the Heavyweight division and lost a unanimous decision to Matt Kovacs at Cage Warrior Combat (CWC) 9 on November 2, 2013.[3]